Look for a junked out A body from 64-72, 67-68 had duel pistons, 69-74 had a single piston, or go to a conversion from a brake vendor such as S/S (Stainless Steel) or try Ames Performance Eng., thay handle Pontiac Parts, All conversions have single piston brakes. But get a manual to help you thru the project.
Good Luck!